The broad goal of this proposal is to develop novel methodologies to achieve site-selective labeling of peptides and peptidomimetics with PET tracers. The novel strategies will be based on the incorporation of bioorthogonally labeled unnatural amino acids via solid phase peptide synthesis protocols. The resulting radiolabeled peptides will be evaluated in parallel, and hit candidates further tested in animal imaging experiments. The candidate, Thomas Reiner, has extensive experience in the quantitative sciences, specifically synthetic organometallic and organic chemistry. He has worked on bioorthogonal strategies before. He will apply his skillset to this translational approach, which i at the interface of organic synthesis, biology and biomedical imaging. The successful development of techniques and protocols which allow site-selective incorporation of bioorthogonal labels into peptides, as well as the parallelized evaluation of their corresponding radiolabeled versions will represent a major step in biomedical imaging research, greatly facilitating design, evaluation, and ultimately clinical translation of diagnosic radiolabeled probes. The long term goal of the candidate is to develop novel and more efficient methodologies which allow conjugation of PET labeled imaging agents to targeted biomolecules. This work will be performed at the Department of Radiology of Memorial Sloan-Kettering Cancer Center under the mentorship of Dr. Jason Lewis. Both Dr. Hedvig Hricak and Dr. Wolfgang Weber, who are experts in preclinical and clinical imaging research, will serve as co-mentors. The members of the advisory committee are fully committed to the mentored research training of the candidate, allowing him to develop a strong and successful career as an independent biomedical researcher. The specific aims of this proposal are to first synthesize tetrazine and trans-cyclooctene amino acids, followed by their incorporation into somatostatin-analogs, generating a library of bioorthogonally labeled peptides. Then, these peptides will be converted into their corresponding 18F, 89Zr and 64Cu labeled versions by utilizing parallel bioorthogonal assembly and purification techniques. High affinity and selectivity peptides will subsequently be tested in animal models of cancer. Here, tetrazine and trans-cyclooctene amino acids will not only allow the fast and selective synthesis, but also the rapid chromatography free purification of radiolabeled peptides, facilitating multiplexed parallel synthesis of radiolabeled peptide libraries. Hit candidates will be evaluated for their performance as targeted probes in animal models of cancer. If successful, these new technologies will allow the rational and high-throughput evaluation of targeted peptidic PET probes, streamlining their development and increasing the chances of successful outcomes. The design of radiolabeled targeted peptides via tetrazines/trans-cyclooctenes could become a standard technique for preclinical biomedical imaging and ultimately clinical research.